Spin injection from ferromagnetic Co nanoclusters into organic semiconducting polymers

We report a spin-polarized hole injection into organic semiconductor poly[2-methoxy-5-($2^{\prime }$-ethylhexyloxy)-1,4-phenylenevinylene (MEHPPV) layer from cobalt (Co) nanoscale clusters based on electroluminescence (EL) studies at liquid nitrogen temperature. The magnetic field effect on EL intensity indicates that the spin injection essentially increases the singlet-to-triplet exciton ratio in organic semiconductors. The Co size-dependent EL shows that the spin-injection efficiency decreases rapidly when the Co clusters percolate to a continuous film. The thickness-dependent EL enhancement of magnetic field suggests that the diffusion length of spin-polarized holes is about $60\mathrm{nm}$ in the MEHPPV.

Figure 1

Schematic device structure and band diagram for $\mathrm{ITO}∕\mathrm{MEHPPV}\left(30\mathrm{nm}\right)∕\mathrm{Co}\left(2.0\mathrm{nm}\right)∕\mathrm{Al}$ LED at reverse bias. The work functions are $5.0\mathrm{eV}$ (Ref. 9) and $4.2\mathrm{eV}$ (Ref. 13) for Co and Al. The ITO work function is assumed to be $4.8\mathrm{eV}$. The electron affinity and bandgap are 2.8 and $2.2\mathrm{eV}$ for MEHPPV (Ref. 27).

Figure 2

(a) EL-voltage-current characteristics in different magnetic fields at reverse bias. (b) The reverse EL (REL), forward EL (FEL), and PL spectra from the MEHPPV LED at 0, 200, and $450\mathrm{Oe}$. The injection current of $100\mathrm{mA}∕{\mathrm{cm}}^2$ and the blue light of $450\mathrm{nm}$ were used as electrical and photoexcitations, respectively. (c) The EL enhancement $[({\mathrm{EL}}_{\text{final}}-{\mathrm{EL}}_{\text{initial}})∕{\mathrm{EL}}_{\text{initial}}]$ as a function of the MEHPPV film thickness at $450\mathrm{Oe}$ and reverse current of $100\mathrm{mA}∕{\mathrm{cm}}^2$. The straight line is added for eye guidance.

Figure 3

EL dependences of magnetic field for the MEHPPV with Co and Au nanoclusters, respectively, at reverse injection current of $80\mathrm{mA}∕{\mathrm{cm}}^2$.

Figure 4

TEM micrographs showing the morphologies of Co electrodes prepared from different nominal thicknesses: (a) 0.8, (b) 2.0, and (c) $4.8\mathrm{nm}$. The dark islands in (a) and (b) are the Co clusters formed on the MEHPPV film. The electron diffraction [inset in (b)] was collected from the Co clusters. The micrograph in (c) shows a continuous Co film (dark layer) formed on MEHPPV thin film (light layer). The magnified micrograph (d) displays the crystalline structure of individual Co clusters.

Figure 5

(a) EL enhancement $[({\mathrm{EL}}_{\text{final}}-{\mathrm{EL}}_{\text{initial}})∕{\mathrm{EL}}_{\text{initial}}]$ as a function of the Co nominal thickness for the LED of $\mathrm{ITO}∕\mathrm{MEHPPV}\left(30\mathrm{nm}\right)∕\mathrm{Co}∕\mathrm{Al}$ at $450\mathrm{Oe}$ and reverse current of $100\mathrm{mA}∕{\mathrm{cm}}^2$. The labeled numbers indicate the respective average cluster sizes. (b) Magnetization hysteresis in low field and high field (inset) for the Co electrodes prepared with different nominal thicknesses.